1. Field of the Invention
The present invention relates to a thermal electron emitter and a plasma generator using the same.
2. Description of the Related Art
In a semiconductor manufacturing process, a process of implanting ions into a semiconductor wafer has been implemented as a standard process for the purpose of changing conductivity, for the purpose of changing a crystal structure of the semiconductor wafer, and for some other purposes. An apparatus used in this process is generally called an ion implanter.
In such an ion implanter, ions are generated by an ion source equipped with an indirectly heated cathode type plasma generator. The indirectly heated cathode type plasma generator heats a filament by allowing an electric current to flow through the filament to generate thermal electrons, and heats a cathode by the thermal electrons. Thermal electrons generated by the heated cathode are accelerated within an arc chamber and collided with source gas molecules in the arc chamber, so that atoms included in the source gas molecules are ionized to generate plasma. Some of ions in the generated plasma reach an outlet hole (front slit) of the arc chamber by diffusion and become an ion beam extracted by an electric field generated by an extracting electrode. Such a cathode for emitting thermal electrons may include, for example, a cathode cap heated by the thermal electrons emitted from the filament, and a tubular member of which an end is attached to the cathode cap.
Since the cap for emitting thermal electrons into the arc chamber is exposed to an internal space of the arc chamber, the cap is sputtered by ions in the plasma generated within the arc chamber and may wear down with the use of the ion source. Since it becomes difficult to stably generate ions as the cap gradually wears down, it is desirable to increase the thickness of the cap in order to extend the lifetime of the ion source. On the other hand, when the thickness of the cap is increased, an input energy amount necessary to sufficiently heat the cap and stably generate thermal electrons increases. When the energy amount necessary to heat the cap increases, the use of a power supply of higher power is needed and therefore, it may have influence on the cost of the ion source.